1. Field of the Invention
The present invention relates to reflow soldering, and more particularly concerns reflow soldering in which tip skid of the tool tip is minimized.
2. Description of Related Art
Reflow soldering is a process for soldering together pre-tinned electrical circuit parts by application of heat and, in many processes, pressure. The heat causes the solder to melt. If the two parts are in good mechanical contact when the solder is melted, the solder from each piece will flow and coalesce. When the solder again solidifies, the two parts will be joined by the solid solder which is between them and wetted to them. Reflow soldering is used extensively in manufacture and repair of printed circuit boards. Typical applications of reflow soldering are the soldering of ribbon cable to printed circuit conductors and soldering of leads of integrated circuit chips to printed circuit conductors. Many different types and configurations of soldering tips are used. In one example of a reflow soldering operation a heater bar or thermode having a width capable of extending over a number of relatively thin, fine leads to be simultaneously soldered, is heated and lowered to contact a plurality of the pre-tinned leads that are to be connected to underlying pre-tinned connecting pads. An electric current is applied to the thermode to raise its temperature and effect flow of the solder of the pre-tinned leads and pads while the thermode is pressed against the leads. After the solder cools the heater bar is raised.
When the thermode is lowered into contact with an unleveled or tilted work surface, one edge initially will contact the work surface so that further lowering of the heater bar to align it with the unleveled or tilted work surface will cause a lateral movement, e.g. skidding, of the heater bar along the work surface. This undesired motion has been termed "tip skid". As leads of electronic devices become more minute, fragile and expensive, even small amounts of thermode tip skid can do major damage. Even minor amounts of unleveling or surface slope can result in damaging tip skid. The damage may be deformation of the leads, displacement, misalignment and consequent electronic failures.
Presently used bonding mechanisms have been arranged for various types of compliant and even pivotal mountings. For example, parallel flexure strips that mount the tool tip to its drive ram allow for vertical compliance, but allow no rotational compliance. This can result in movement of the leads, unequal pressure across the thermode and difficult set-up problems. Such a parallel flexure mounting is shown in U.S. Pat. No. 3,238,351 to W. H. Hill for Electrode Assembly Having Cantilever Suspended Electrodes. Angled compliant flexure designs, such as shown in the patent to Hill, U.S. Pat. No. 4,598,853 for Open Center Flexural Pivot Wire Bonding Head, effectively provide a virtual pivot point about the center of the heater bar but may induce distortion in the mechanism and the leads. With the pivot point effectively positioned at the center of the heater bar, tip skid still occurs and the degree of compliance in such a mechanism is limited. Virtual pivot flexures may add rotational compliance about the center of the face of the thermode, resulting in unequal pressure distribution and may cause movement of leads during rotational compliance.
Some systems employ a half bearing above the thermode, having a radius equivalent to the distance from the thermode to the bearing center. This effectively provides a virtual pivot about the center of the thermode but is not suitable for small thermodes or peg tips. Further, such mechanism is sensitive to friction, contamination and part wear.
U.S. Pat. No. 4,871,899 for Tiltable Electric Thermode for Multiple Connection Reflow Soldering employs a thermode mounted about a pivotal axis displaced well above the thermode tip and thus is subject to significant amounts of tip skid to accommodate rotation of the tip about the axis.
Accordingly, it is an object of the present invention to accomplish bonding in a manner that avoids or minimizes above mentioned problems.